The design and construction of a high speed, brushless DC motor for use as a compressor motor gives rise to a number of problems.
Such a motor must produce sufficient power to be effective as a compressor motor over a range of loads. To minimize the combined volume of the motor and compressor it is desirable to design a high speed, low torque motor of high efficiency.
High speed, brushless DC motors are known. In this connection. reference can be made to the text "Direct Current Machines", by Say and Taylor, 2nd Edition (Pitman International), and particularly sections 4.8, 5.10 and 11.4. Such motors are generally of a lower power output than that required for a refrigerant compressor motor. One form of such a motor uses a rare earth magnet (in particular, a grade of Neodymium-Iron-Boron [NdFeB]). This material, however, and other rare earth magnetic material, generally has a low tensile strength and is somewhat brittle. This has restricted its use to relatively lower powered high speed electric motors which are unsuitable for compressor motors.
In a refrigeration compressor application using the new refrigerants now available, such as R134A and other CFC refrigerants which are considered environmentally safe, it is not possible to use lubricants which are chemically incompatible with the refrigerant. This may exclude the use of conventional bearings in the compressor and hence the desire to be able to use non-contact bearings like magnetic or foil bearing. Such bearings would also, therefore, be used in the driving motor.
It is an object of the present invention to design a high speed electric motor which has a relatively high power output and a very high power to volume ratio.
It is also an object of the invention to design an electric motor able to be used for a refrigerant compressor, the rotor preferably being supported by oilless or non-contact bearings, such as magnetic bearings, foil bearings or the like but also able to be used with standard bearings in different applications.
It is also desirable to provide an electric motor for use as a compressor motor and the control of which is able to be effected by relatively low cost control circuitry.
It is also desirable to provide an electric motor the rotor of which incorporates a rare earth magnet in a structure which provides the necessary mechanical strength.